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The Galilean moons of Jupiter
3.5 g/cc 3.0 g/cc 1.9 g/cc 1.8 g/cc
Last time …. Io
• Innermost Galilean moon• Heavy tidal heating – orbital
resonance• Active Sulfur volcanos
Europa
Water Ice surface!
Most recent estimate: crust ~ 19 km thick
(Structure: gravity and magnetic field data)NOT REAL
GanymedeNASA 2014
(Structure: gravity and magnetic field data + Keck scope measurements of Aurora shifts)
Callisto
Parts of surface quite old
Galilean satellites:
• Io: Volcanically active!
Tidal heating from Jupiter drives the active geology. The origin of the tidal effect is its eccentric orbit, caused by orbital resonance with Europa and Ganymede.
• Europa, Ganymede and Callisto: Ice tectonics!
All three are likely to have internal oceans of water.
On Ganymede and Callisto the source of heating is somewhat of a mystery (tidal effects not enough).
These three moons combined may contain more liquid water than earth!!
Titan
Natural color/smog (Organo-Nitrogen)
1.9 g/cc
Dense atmosphere !1.4 x earth95% N2
5% CH4 , etc.
Thick atmo.Used to be thought of as biggest moon, now Ganymede is
bigger.
Cassini IR/false color
KrakenMare
?
‘Shangri-La’
plains/desert of dark smog
particles
Icy highlands
Adiri as seen by Huygens
Kraken mare
Ligeia mare
Methane Lakes!!
Transient feature in Ligeia mare!
• Waves?• Gasses released?• Summer Buoyant solids?
Huygens's descent
Images are flattened (360° Mercator)
Landing site: dry (methane) river bed.
Jovian rings
Saturn thru a bad scope
“I discovered another very strange wonder, which I should like to make known to their Highnesses . . . , keeping it secret, however, until the time when my work is published . . . . the star of Saturn is not a single star, but is a composite of three, which almost touch each other, never change or move relative to each other, and are arranged in a row along the zodiac, the middle one being three times larger than the lateral ones, and they are situated in this form:
oOo.”
Galileo, 1610
Christiaan Huygens, Opera varia (Lyon, 1724
Rings are a common feature of ALL Jovian worlds!
Jupiter
Keck
Galileo from the backside
Jupiter
Hubble
Uranus
Neptune
Saturn’s rings
A
B C
Cassini
Encke
Major features:
GAPS BANDS
APOD 7/23/04
Saturn's Rings in Natural Color
Mostly water-ice reflecting sunlight …• Thinner portions darker (Like the gaps)• Some unknown dark dirt ….• Outgassing of trace elements
Why the colors?
Mostly frozen water Ring thickness: “tens of meters”
Particles sizes: 1 cm – 10 m (10 cm most abundant)
Est. total mass ≈ 3 × 1019 kg ( ≈ ¾ Mimas )
Saturn’s rings:
Relationship to moons and rings:
What causes all the structure?
Moons do a lot …
• Resonance orbits cause gaps• Sheppard moons can confine/concentrate rings• Moons can simply plow gaps• Moons can add material
A) Orbital resonance
(remember Io?)
At what radius would something orbit at to have half the period of Mimas? 22
2
12 Pp
34
13 RCrC
RRr 630.034
1
(0.630) × (185,540 km) = 117,000 km
Inner edge ofCassini division:
Huygens gap
Mimas’ orbit
Mimas
2:1 resonanceorbit
Every 2nd orbit, each bit of dust in the Cassini orbit gets an outward kick at same place.
The A Ring's outer edge is maintained by a destabilizing 7:6 resonance with the moon Janus.
B) Sheppard moons
Sheppard movie (http://www.youtube.com/watch?v=fdUlpeUFfxI&NR=1&feature=fvwp)
The Encke and Keeler gaps within the A Ring are cleared by the embedded
moonlets Pan and Daphnis, respectively:
C) Moons can clear gaps in rings
( 1:1 resonances )
Daphnis churns up the edges of the Keeler Gap
Daphnis: 8 kmKeeler gap: 42 km
The Encke gap and Pan
Pan: 14 kmEncke gap: 325 km Close-up of the Encke Gap. The central
ringlet is coincident with Pan's orbit. (Wik)
D) Moons can also be the source of rings
Enceladus and E-Ring
Black dotEnceladus
Plume jets from the moon Enceladus supplying vapor to the E ring
Cryo-volcanic plumes on Enceladus
Imaginative picture of the day:
Anthe partial Ring (20⁰ long.)
Anthe (10:11) and Methone (14:15) resonance with Mimas
micrometeoroid impacts?
Ring Arcs
Lot’s of unknown processes…
Streamer channels caused by Prometheus in the F-ring
Ring spokes?
Ring origins?
Most other sources:
Our book: • Ring particles can’t be old, they’re constantly being worn down
• New particles must be constantly created from moonlets
• Moonlets big enough to survive 4.5 billion years of sandblasting
• Rings are old, remnants of the early days
• Tidal forces of big planets …
Keeps/kept particles from forming moonsBroke up larger proto-moons that got too close
The Roche Limit
An object held together only by gravitational forces –
Unstable within Roche’s limit
Objects held together by chemical forces (ice/rock) are tougher
(Tuft’s)
